Cervical cancer is the fifth most deadly cancer in women. Worldwide, approximately 500,000 cases of cervical cancer are diagnosed and about 250,000 women die from this disease annually (www.who.int/mediacentre/factsheets).
Most (80-90%) invasive cervical cancer develops in flat, scaly surface cells that line the cervix (called squamous cell carcinomas, SCC). Approximately 10-15% of cases develop in glandular surface cells (called adenocarcinomas, AdC). Less commonly, cervical cancers have features of both SCC and AdC. These are called adenosquamous carcinomas or mixed carcinomas (www.cancer.org).
During the process of cervical cancer development, normal cervical cells gradually develop pre-cancerous changes that turn into cancer. Cervical cancer evolves from pre-existing noninvasive premalignant lesions referred to as cervical intraepithelial neoplasias (CINs), ranging from CIN I (mild dysplasia) to CIN II (moderate dysplasia) to CIN III (severe dysplasia/carcinoma in situ). This process usually takes several years but sometimes can happen in less than a year. For most women, pre-cancerous cells will remain unchanged and disappear without any treatment.
Screening for malignant and premalignant disorders of the cervix is usually performed according to the Papanicolaou (PAP) system. The cervical smears are examined by light microscopy and the specimens containing morphologically abnormal cells are classified into PAP I to V, at a scale of increasing severity of the lesion. But, present PAP test has some limitations and is not completely ideal for screening as it suffers from suboptimal single-test sensitivity, limited reproducibility, and many equivocal.
There is a strong association between certain subtypes of the Human Papillomavirus (HPV) and cervical cancer. Studies have shown that only high-risk HPV types are involved in the progression from cytological normal cervix cells to high grade squamous intraepithelial lesions. Around 15 high-risk (cancer-causing) HPV types have been identified. Although it has been suggested that high-risk HPV testing may improve cervical cancer screening, the specificity for high grade cervical neoplasia of high risk HPV testing is relatively low. This low specificity of HPV testing leads to a higher number of unnecessarily follow-up diagnostic workups (e.g. colposcopy) and unnecessarily treatment with cryotherapy or loop electrosurgical excision procedure, which permanently alters the cervix and have unknown consequences on fertility and pregnancy.
To improve early detection, the combination of HPV and PAP tests is now approved by the FDA for screening women 30 years of age and older. However, co-testing substantially increases the cost of screening.
In the meanwhile, vaccines for preventing cervical cancer have been developed and one has already been approved by the FDA. But, immunization will only protect against HPV types that are targeted by the vaccine; protection will not be absolute and its longevity is uncertain; as yet, the possibility of genotype replacement cannot be excluded; and older women not covered by vaccination programs will continue to be at risk. Therefore, cervical screening will still be required for control.
Cancer biomarkers have been described in literature and aberrant methylation of genes has been linked to cervical cancer (Virmani et al, 2001). In addition, methylation markers may serve for predictive purposes as they often reflect the sensitivity to therapy or duration of patient survival.
DNA methylation is a chemical modification of DNA performed by enzymes called methyltransferases, in which a methyl group (m) is added to certain cytosines (C) of DNA. This non-mutational (epigenetic) process (mC) is a critical factor in gene expression regulation. (See J. G. Herman, Seminars in Cancer Biology, 9: 359-67, 1999).
An early diagnosis is critical for the successful treatment of many types of cancer, including cervical cancer. If the exact methylation profiles of cervical tumors are available and drugs targeting the specific genes are obtainable, then the treatment of cervical cancer could be more focused and rational. Therefore, the detection and mapping of novel methylation markers is an essential step towards improvement of cervical cancer prevention, screening, and treatment. Thus, there is a continuing need in the art to identify methylation markers that can be used for improved assessment of cervical cancer.